LUDLUM 375 Series User manual

LUDLUM MODELS 375 (INCLUDING SERIES ONE),
375/2, 375/4, AND 375WT
DIGITAL WALL-MOUNT AREA MONITORS
May 2021
Serial Number 179939 and Succeeding
Serial Numbers
Legacy Version

LUDLUM MODELS 375 (INCLUDING SERIES ONE),
375/2, 375/4, AND 375WT
DIGITAL WALL-MOUNT AREA MONITORS
May 2021
Serial Number 179939 and Succeeding
Serial Numbers
Legacy Version

Ludlum Measurements, Inc. May 2021
STATEMENT OF WARRANTY
Ludlum Measurements, Inc. warrants the products covered in this manual to be free of
defects due to workmanship, material, and design for a period of twelve months from
the date of delivery. The calibration of a product is warranted to be within its
specified accuracy limits at the time of shipment. In the event of instrument failure,
notify Ludlum Measurements to determine if repair, recalibration, or replacement is
required.
This warranty excludes the replacement of photomultiplier tubes, G-M and
proportional tubes, and scintillation crystals which are broken due to excessive
physical abuse or used for purposes other than intended.
There are no warranties, express or implied, including without limitation any implied
warranty of merchantability or fitness, which extend beyond the description of the
face there of. If the product does not perform as warranted herein, purchaser’s sole
remedy shall be repair or replacement, at the option of Ludlum Measurements. In no
event will Ludlum Measurements be liable for damages, lost revenue, lost wages, or
any other incidental or consequential damages, arising from the purchase, use, or
inability to use product.
RETURN OF GOODS TO MANUFACTURER
If equipment needs to be returned to Ludlum Measurements, Inc. for repair or calibration, please
send to the address below. All shipments should include documentation containing return shipping
address, customer name, telephone number, description of service requested, and all other necessary
information. Your cooperation will expedite the return of your equipment.
LUDLUM MEASUREMENTS, INC.
ATTN: REPAIR DEPARTMENT
501 OAK STREET
SWEETWATER, TX 79556
800-622-0828 325-235-5494
FAX [phone]

Ludlum Measurements, Inc. May 2021
Table of Contents
Introduction 1
Getting Started 2
External Detector (Option) 2-1
Power Up 2-1
Radiation Units 2-2
*Model 375 Series One 2-2
Checking Parameters 2-3
Setting Alarm-points 2-3
Operational Check 2-4
Specifications 3
Operator Controls and Setup 4
Calibration Controls 4-1
Dipswitch (under calibration cover) 4-2
RS-232 Output 4-2
9-Pin Data Connector 4-3
Typical Internal Detector Setups 4-3
Common Options and Modifications 5
Time and Date Stamp Option 5-1
Relay Options 5-2
Sigma Alarm Modification Option 5-3
4 to 20 mA Isolated Output Driver Option 5-4
Waste Truck (Model 375WT) Modification 5-6
Safety Considerations 6
Environmental Conditions for Normal Use 6-1
Cleaning Instructions and Precautions 6-1
Warning Markings and Symbols 6-2
Replacement of Main Fuse 6-3

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Detector Connector 6-3
Battery Replacement 6-3
Calibration 7
High Voltage 7-1
Calibration Parameters 7-1
Analog Output 7-2
Discriminator 7-2
Battery Charge 7-2
Recycling 8
Parts List 9
Model 375 Digital Wall-Mount Area Monitor without Detector 9-1
Model 375/2 Digital Wall-Mount Area Monitor 9-1
Model 375/4 Digital Wall-Mount Area Monitor 9-1
Model 375 Series One without Detector 9-1
Model 375/2 Series One 9-1
Model 375/4 Series One 9-1
Model 375WT without Detector 9-1
Main Board, Drawing 396 × 160 9-1
Wiring Diagram, Drawing 396 × 176 9-5
Internal Detectors 9-5
Drawings and Diagrams 10

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Introduction
he Model 375 Digital Wall-Mount Area Monitor is designed for
visibility and ease of use. Featuring a wall-mount chassis, the Model
375 has a four-digit LED display that is readable from thirty feet away.
Backlit indicators warn of low radiation alarm (yellow), high radiation
alarm (red), instrument failure (red) and low battery (yellow). A green status
light is a positive indication of instrument operation.
Parameters are protected under a calibration cover. Calibration is easily
accomplished by moving the cal dipswitch to the right, and using the
pushbuttons to increment or decrement the calibration constant, dead time
correction and alarm point parameters. Parameters are stored in non-volatile
memory (retained even with power disconnected).
A five-decade logarithmic analog output is provided. A battery backup provides
48 hours of additional use after the primary power is removed. Depending on
the needs of the customer, the Model 375 comes equipped with an internal
detector or a connector for use with an external detector.
Two common Model 375 systems with internal detectors are:
The Model 375/2 with an internal LND 71210 detector.
The Model 375/4 with an internal LND 71412 detector.
The Ludlum Model 375 Series One is a Model 375 with modified firmware.
Changes include certain display values in decimal points and units. Details can
be found in sections ˝Getting Started˝and ˝Specifications˝.
The Ludlum Model 375WT- Digital Waste Truck Monitor is a Model 375
which has been modified to operate on 12 Vdc. The modification is detailed in
the ˝Common Options and Modifications˝section of this manual.
Section
1
T

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Getting Started
he Model 375 Digital Wall-Mount Area Monitor is designed for
ease of use. This section of the manual is designed to help the first-
time user get started. Initial power-up and basic features of the
Model 375 will be discussed in this section. Other sections of the
manual provide more detailed information.
External Detector (Option)
Warning!
Potential electrical shock hazard - Do not touch the center
pin of the detector connector unless the unit has turned off
and power has been removed for at least 1 minute!
The Model 375 comes equipped with either an internal detector or a
connector for use with an external detector. If your Model 375 is equipped
with a connector on the bottom side of the chassis, an external detector is
required. If you have an external detector, use the cable provided to connect
it to the Model 375.
Note:
Splicing or re-terminating cables must be done carefully.
Improper termination will result in the “shorting-out” of the
detector voltage, a DET FAIL and/or blown-fuse condition.
Power Up
Plug the power cord into a suitable wall (Mains) outlet (see note on
following page).
Section
2
T

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Note:
Model 375 units will normally be wired internally for 120
VAC. If requested, the unit may be wired for 220 VAC.
Check the label next to the AC input receptacle to verify the
required input voltage.
Caution!
Verify instrument voltage input rating before connecting to a
power converter. If the wrong power converter is used, the
instrument and/or power converter could be damaged.
If the RS-232 feature is used, plug in a suitably wired 9-pin
connector cable. (See Page 4-4 for the pin assignment of the 9-pin
connector.) Turn power ON with the left side panel switch. Do
not turn power OFF unless the unit is to be removed from service.
Read and then remove the sticker (illustrated to the left) from the
instrument calibration cover. Checking and setting of the alarm-
point(s) is discussed in detail below and on pages 2-3 and 7-1 of
this manual.
Initial power-up will momentarily activate the internal front panel lights,
sound the audio, and display "8888" on the 4-digit LED display. The
firmware version number (39665Nyy) is then displayed as "396" and "65yy"
(where yy represents the current version number).
When the instrument has finished measuring background, it will display the
current radiation reading and begin checking for an alarm condition.
Radiation Units
The Model 375 may be calibrated for almost any desired radiation units of
measure. Common units of measure include mR/hr, µR/hr, R/hr, mSv/h,
µSv/h, cps, cpm, and kcpm. In each case, the unit of measure is indicated
underneath the four-digit display. Throughout the rest of this manual, the
notation <units> will be used as a substitute.

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* Model 375 Series One: In the special case of the Model 375 Series One
make note of the following changes resulting from firmware modification:
With the CAL dipswitch in the left position (normal operating mode),
two decimal places will be displayed instead of one.
With the CAL dipswitch in the right position (calibration mode), only one
decimal place will be displayed.
Calibration constant will be displayed in cps per unit, instead of cpm per
unit.
Dead Time Correction will display in 10thsof microseconds (i.e. 50 µsec
instead of 5 µsec).
Checking Parameters
Check the low alarm point setting by pressing the LOW ALARM button. The
low alarm point will be displayed as long as the button is pressed. The low
alarm point is in units of <units>. The low alarm point can be set from 0.1
<units> to 9999 <units>*.
Check the high alarm point setting by pressing the HIGH ALARM button. The
high alarm point will be displayed as long as the button is pressed. The high
alarm point is in units of <units>. The high alarm point can be set from 0.1
<units> to 9999 <units>*.
Check the calibration constant by pressing the CAL CONST button. The
calibration constant will be displayed as long as the button is pressed. The
calibration constant is in units of cpm (counts per minute) per <units>*.
The calibration constant can be set from 0.1 cpm/<units> to 9999
cpm/<units>*.
Check the detector dead time correction by pressing down on the DEAD
TIME button. The dead time correction will be displayed as long as the
button is pressed. The dead time correction is in units of microseconds*.
The dead time correction can be set from 0.1 microseconds to 9999 micro-
seconds*.
Setting Alarm-points
The LOW ALARM and HIGH ALARM points can only be changed while the
instrument is in calibration mode. Switch the top dipswitch CAL MODE

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(behind the calibration cover) to the right to place the instrument into
calibration mode.
Changing alarm-points is done by holding down the corresponding
parameter key and pressing the up or down arrow buttons. Alarm-points can
be set in the range of 0.1 to 9999*. When an alarm-point is changed the
instrument will sound an audible beep to confirm the saving of the
parameter, and will then return to displaying the current radiation level.
Note:
Once the alarm point(s) is set, it is important to remember to
switch the CAL MODE switch back to the left. This action
protects the parameters from inadvertent changes.
Operational Check (optional)
The operational check is an important assurance that the radiation detector
and electronics are working correctly.
Note:
Ludlum Measurements suggests that an operational check be
performed on a regular basis. Local procedures may
supersede this suggestion.
For an operational check it is necessary to use a radiation check source (not
included, but available). When not being used, store the check source in a
secure area.

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Note:
LMI check sources present very minimal risks and are
therefore unlicensed (Exempt Quantity Sources reference:
10 CFR 30.71 Schedule B). The radioactive element is
sealed (permanently bonded or fixed inside a capsule) so
you need not wash your hands after handling. Radiation
exposure while handling this source is very minimal with
no identified long or short term risks. Although the
amount of radiation given off by exempt sources is so low
that it presents no significant hazard, they should be
handled with care and respect. Time, distance and shielding
are the best ways to control exposure.
1. Taking the source in hand, place it so that it is located on or near
the center (same location each time) of the detector. (For internal
detector models, a metal plate on the bottom of the chassis
identifies the location of the internal detector.) Hold it there for
approximately 5 seconds or until the reading stabilizes. Take note
of the displayed level of radiation.
2. Verify that the reading is within 20% of the last reading
obtained. Remove the source from the detector.
3. If an alarm is activated, ensure that all visual and audible devices
(if applicable) work correctly.

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Specifications
Compatible Detectors: G-M, proportional and scintillation.
Display: 4-digit LED display with 0.8" (2 cm) character height.
Display Range: 000.0-9999*.
Display Units: Can be made to display in µR/hr, mR/hr, R/hr, µSv/h,
mSv/h, Sv/h, µrem/hr, mrem/hr, rem/hr, cpm, cps, and others.
Linearity: Readings within 10% of true value with detector connected.
Operating Range: Depends upon the type of detector used. The
operating range of the 375/2 is from 0.1 mR/hr to 1 R/hr. The operating
range of the 375/4 is from 1 mR/hr to 8 R/hr. External detectors will
have different operating ranges.
Response: Typically 3 seconds from 10% to 90% of final reading.
Status (green light): Indicates the instrument is functioning properly.
Low Alarm: Indicated by a yellow light and slow beep (1 per second)
audible tone (can be set at any point from 0.0-9999*).
High Alarm: Indicated by a red light and a fast beep (4 per second)
audible tone (can be set at any point from 0.0-9999*).
Note:
Audible indicators can be configured as a single beep if
desired. Audio intensity is controlled by rotating the baffle
on the audio device.
DET Fail: Indicated by a red light and an audible tone greater than 68dB
at 2 feet for conditions of detector overload, no count from detector or
instrument failure.
Section
3

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Low bat: Indicated by a yellow light, beginning when 2 hours of battery
life remain.
Connector: Series ˝C˝(others available).
Calibration Controls: Accessible from the front of instrument (protective
cover provided).
High Voltage: Adjustable from 200-2500 volts.
Dead Time: Adjustable to compensate for dead time of the detector and
electronics (can be read on the display).
Overload: A display reading of ˝-OL-˝and audible FAIL alarm indicate
detector saturation. Usually set to initiate just above the highest range of
the detector.
Over-range: A display reading of ˝----˝ and activated low and high alarms
indicate that the radiation field being measured has exceeded the counting
range of the instrument (or when dead time correction accounts for more
than 75% of the displayed reading).
Data Output: A 9-pin connector providing 5-decade log output, RS-232
output, signal ground connection, FAIL and HIGH ALARM signals (current
sink) and direct connection to battery and ground.
RS-232 Output: A 2-second dump for computer data logging.
Remote (optional): Ludlum Model 271 or 272 remote units.
Power: 95-135 VAC (178-240 VAC available), 50-60 Hz single phase
(less than 100 mA), 6 volt sealed lead acid rechargeable backup battery
(built-in).
Battery Life: Typically 48 hours in non-alarm condition; 12 hours in
alarm condition.
Battery Charger: Battery is continuously trickle charged when the
instrument is connected to line power and turned on.
Warning!:
Only certified technician or calibration personnel should
replace battery.

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Construction: Aluminum housing with ivory powder-coat finish.
Size: 9.7" (24.6 cm) W × 7.4" (18.7 cm) H × 2.5" (6.4 cm) D
Weight: 5.2 lb. (2.4 kg)

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Operator Controls and Setup
Calibration Controls
Remove the calibration cover to expose the calibration controls. The
calibration controls include the up/down buttons, five calibration
potentiometers, and the option dipswitch (detailed in the following
subsection). The five potentiometers are detailed below.
Warning!
Do not touch the circuit board in the calibration window
due to potential for electric shock.
ANALOG: Used to adjust the logarithmic analog voltage output.
Adjusted in calibration mode to the full-scale voltage reading or adjusted
to a known point at some given reading.
HV: Used to set the high voltage required for detector operation.
Adjustable from 0-2500 Vdc. The high voltage required will depend on
the type of detector used. Internal GM detectors typically require 550
Vdc. Be sure to check the high voltage with a high impedance (1000-
Mohm impedance) voltmeter only. A high voltage checkpoint is located
next to the HV potentiometer.
DISC: Internal discriminator used to set pulse threshold for counting
pulses from the detector. Pad allows direct measurement of threshold
voltage. Utilize a Ludlum Model 500 Pulser or equivalent to inject pulses
of the desired threshold size. The pulse height threshold is adjustable
from 2.0 mVdc to 100 mVdc.
BAT CHARGE: Used to set the backup battery trickle charging voltage. It
is set to 6.9 Vdc while the battery is disconnected.
OVERLOAD: Used to set the detector current overload point. When
excessive radiation causes the detector to overload, this set point will
cause the FAIL light to engage, and the display will be forced to -OL-.
Section
4

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Dipswitch (under calibration cover)
When the calibration cover is removed, a four-pole dipswitch is accessible
that can activate or deactivate options. These four options are CAL MODE,
LATCH, ALARM, RANGE and SINGLE BEEP.
Dipswitch 1: Switching the top CAL MODE switch to the right places the
instrument into calibration mode. Parameters can only be changed while
the instrument is in calibration mode. Calibration mode also changes the
analog output to full-scale so that the full-scale voltage may be set by the
ANALOG potentiometer. Calibration mode also slows the response time
of the display and increases the accuracy. If the display seems too erratic,
leaving this switch in the calibration mode during operation will help.
Moving the CAL MODE switch back to the left locks the parameters and
disables any further changes.
Dipswitch 2: The second switch, LATCH ALARM, changes the high
alarm to a latching alarm. This switch does not affect the low alarm,
which is always non-latching. When switched to the left, the high alarm
is non-latching; the alarm automatically turns off when the radiation
level drops below the alarm point. When switched to the right, the high
alarm light and audio signals are latched until either the LOW ALARM or
HIGH ALARM button is pressed.
Dipswitch 3: The third switch, RANGE, selects the range of the
instrument. To select the 0.1 <units> - 999.9 <units>* range, switch the
RANGE switch to the left. To select the 1 <units> - 9999 <units>*
range, switch the RANGE switch to the right.
Dipswitch 4: Switching the fourth switch to the right places the
instrument into SINGLE-BEEP mode. This option limits the audio
output to a single half-second beep on LOW ALARM and HIGH ALARM.
DET FAIL audio output (steady tone) is not limited.
RS-232 Output
With the CAL MODE dipswitch in the left position the Model 375 dumps RS-
232 data onto pin 4 of the 9-pin connector every two seconds. The
communication parameters are 2400 baud, 8 data bits, no parity, 1 stop bit,
and no hardware handshaking.

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9-Pin Data Connector
The 9-pin connector provides output signals from the instrument and
input voltage to the instrument. The pin assignments are:
pin1- +BATTERY
pin2- GND IN
pin3- FAIL_L
pin4- RS232 DUMP
pin5- ANALOG OUT
pin6- NA
pin7- HIGH ALARM_L
pin8- EXT RESET_L
pin9- +5VDC OUT
The FAIL and HIGH ALARM digital signal outputs are open drain 2N7002
outputs, able to sink about 50 mA each.
Typical Internal Detector Setups
Model 375/2
Typical response and set points for the model 375 with internal energy-
compensated LND 71210 GM detector is as follows:
Operating Voltage: 550 Vdc
Threshold: 100 mVdc
Calibration Constant: 1000 cpm/mR/hr
Dead Time Correction: 30 µsec-150 µsec
Linear Range with DTC: 0.1 mR/hr - 1 R/hr
Typical Checkpoints:
1 mR/hr
4 mR/hr
8 mR/hr - calibration constant set point
40 mR/hr
80 mR/hr
400 mR/hr
800 mR/hr - dead time correction set point
1,000 mR/hr